EEE 2103 Circuit Theory I

Course Content

CHAPTER I: Circuit Variables

• 1.1 Electrical Engineering: An Overview

• 1.2 The International System of Units

• 1.3 Circuit Analysis: An Overview

• 1.4 Voltage and Current

• 1.5 The Ideal Basic Circuit Element

• 1.6 Power and Energy

1.1 Electrical Engineering: An Overview

• Electrical engineering is the profession concerned with systems that

produce, transmit, and measure electric signals.

• Electrical engineering combines the physicist’s models of natural

phenomena with the mathematician’s tools for manipulating those models

to produce systems that meet practical needs.

• Electrical systems pervade our lives; they are found in homes, schools,

workplaces, and transportation vehicles everywhere.

CLASSIFICATION OF ELECTRIC SYSTEMS

• Five major classifications of electrical systems:

– Communication systems

– Computer systems

– Control systems

– Power systems

– Signal-processing systems

Communication systems

• Communication systems are electrical

systems that generate, transmit, and distribute

information. Well-known examples include

television equipment, such as cameras,

transmitters, receivers, and VCRs; radio

telescopes, used to explore the universe;

satellite systems, which return images of

other planets and our own; radar systems,

used to coordinate plane flights; and

telephone systems.

Figure 1.1 A telephone system.

Computer systems

• Computer systems use electric signals to process information ranging from

word processing to mathematical computations.

• Systems range in size and power from pocket calculators to personal

computers to supercomputers that perform such complex tasks as

processing weather data and modeling chemical interactions of complex

organic molecules.

• These systems include networks of microcircuits, or integrated circuits

postage-stamp sized assemblies of hundreds, thousands, or millions of

electrical components that often operate at speeds and power levels close to

fundamental physical limits, including the speed of light and the

thermodynamic laws.

An exemplary study uses computer systems to investigate ligand-

enzyme interactions by Fehmi Bardak et al.

Control systems

• Control systems use electric signals to regulate processes.

• Examples include the control of temperatures, pressures, and flow rates in

an oil refinery;

– the fuel-air mixture in a fuel-injected automobile engine;

– mechanisms such as the motors, doors, and lights in elevators; and the

locks in the Panama Canal.

– The autopilot and auto landing systems that help to fly and land

airplanes are also familiar control systems.

Power systems

• Power systems generate and distribute electric power.

• Electric power, which is the foundation of our technology-based society,

usually is generated in large quantities by nuclear, hydroelectric, and

thermal (coal-, oil-, or gas-fired) generators.

• Power is distributed by a grid of conductors that crisscross the country. A

major challenge in designing and operating such a system is to provide

sufficient redundancy and control so that failure of any piece of equipment

does not leave a city, state, or region completely without power.

Signal-processing systems

• Signal-processing systems act on electric signals that represent information.

• They transform the signals and the information contained in them into a more

suitable form. There are many different ways to process the signals and their

information.

• For example, image-processing systems gather massive quantities of data from

orbiting weather satellites, reduce the amount of data to a manageable level, and

transform the remaining data into a video image for the evening news broadcast.

• A computerized tomography (CT) scan is another example of an image-processing

system. It takes signals generated by a special X-ray machine and transforms them

into an image such as the one in Fig. 1.2. Although the original X-ray signals are of

little use to a physician, once they are processed into a recognizable image the

information they contain can be used in the diagnosis of disease and injury.

Figure 1.2

A CT scan of an adult head.

Interaction between engineering disciplines

• Although an engineer is interested in a

specific area, he/she must also be

knowledgeable in the other areas that

are related to subject of interest.

• The fundamental rules and circuit

analysis techniques that will be

learned throughout this course applies

to all fields mentioned so far.

Figure 1.3 An airplane.

Circuit Theory

• An electric circuit is a mathematical model that approximates the behavior of an

actual electrical system.

• The models, the mathematical techniques, and the language of circuit theory will

form the intellectual framework for your future engineering endeavors.

• Circuit theory is a special case of electromagnetic field theory: the study of static

and moving electric charges. Although generalized field theory might seem to be an

appropriate starting point for investigating electric signals, its application is not

only cumbersome but also requires the use of advanced mathematics. Consequently,

a course in electromagnetic field theory is not a prerequisite to understanding the

material in this book. We do, however, assume that you have had an introductory

physics course in which electrical and magnetic phenomena were discussed.

Assumptions made in circuit theory

• Three basic assumptions permit us to use circuit theory, rather than

electromagnetic field theory, to study a physical system represented by an

electric circuit. These assumptions are as follows:

• Electrical effects happen instantaneously throughout a system.

• The net charge on every component in the system is always zero.

• There is no magnetic coupling between the components in a system.

1.2 The International System of Units

1.3 Circuit Analysis: An Overview

• A commonly used mathematical

model for electrical systems is a

circuit model.

• The elements that comprise the circuit

model are called ideal circuit

components.

• The physical prototype is an actual

electrical system, constructed from

actual electrical components.

1.4 Voltage and Current

• Voltage is the energy per unit charge created by the separation.

• The rate of charge flow is known as the electric current,

• Although current is made up of discrete, moving electrons, we do not need

to consider them individually because of the enormous number of them.

Rather, we can think of electrons and their corresponding charge as one

smoothly flowing entity. Thus, i is treated as a continuous variable.

1.4 Voltage and Current

1.5 The Ideal Basic Circuit Element

• An ideal basic circuit element has three attributes:

– (1) it has only two terminals, which are points of connection to

other circuit components;

– (2) it is described mathematically in terms of current and/or

voltage; and

– (3) it cannot be subdivided into other elements.

Passive sign convention

Whenever the reference direction for the current in an element is

in the direction of the reference voltage drop across the element

(as in Fig. 1.5), use a positive sign in any expression that relates

the voltage to the current. Otherwise, use a negative sign.

1.6 Power and Energy

• Power is energy per unit time.

1.6 Power and Energy

Polarity references and the expression for power.